Shift alertness calculator
Updated October 8, 2026 · 2 min read · free calculator
This calculator predicts how alert you are likely to be through each shift, using the three-process model of alertness and sleep it assumes from your schedule. It draws the curve, marks where it falls below 7, the model's sleepiness line, and lists each shift's lowest point with an approximate KSS score.
Predict your alertness
Calculator · runs in your browser, nothing is sent
An end time earlier than the start means the shift ends the next day.
- Shifts
- Assumed sleep
- Predicted alertness
- Sleepiness line (7)
| Shift | Lowest | KSS about | Sleep in the 24 h before | Awake at start |
|---|---|---|---|---|
| Mon, Nov 2 23:00 to 07:00 | 5.0 at 05:45, below 7 | 7.6 | 8 h | 16 h |
| Tue, Nov 3 23:00 to 07:00 | 4.2 at 05:45, below 7 | 8.1 | 4.5 h | 10.5 h |
| Wed, Nov 4 23:00 to 07:00 | 3.8 at 05:30, below 7 | 8.3 | 4.5 h | 10.5 h |
| Fri, Nov 6 07:00 to 15:00 | 9.3 at 07:00 | 5.0 | 11.5 h | 1 h |
A model estimate from assumed sleep, not a measurement of how you slept or feel. Naps are not modeled.
The example is three nights from 23:00 to 07:00, then a 07:00 to 15:00 day shift that starts 24 hours after the last night ends. Edit the shifts to match yours.
What you enter
- Shifts: date, start and end. An end earlier than the start means the shift ends the next day.
- Your usual bedtime, from 21:00 to 00:00.
- The sleep you need, from 7 to 9 hours.
- Morning or evening type, which moves the body clock 1 or 2 hours earlier or later.
What it assumes about sleep
- You wake at 07:00 the day before the first shift, after a full night's sleep.
- You go to bed at your usual time, or 1 hour after a shift that ends later.
- Sleep that starts between 06:00 and 12:00 lasts 4.5 hours, as after a night shift. Other sleep lasts the full amount you need.
- You are up at least 1 hour before the next shift.
- Naps are not included.
The 4.5-hour day sleep and the bedtime 1 hour after a night shift come from the published model.
Worked example: three nights
| Shift | Lowest | KSS about | Sleep in 24 h before | Awake at start |
|---|---|---|---|---|
| Night 1, 23:00 to 07:00 | 5.0 at 05:45 | 7.6 | 8 h | 16 h |
| Night 2 | 4.2 at 05:45 | 8.1 | 4.5 h | 10.5 h |
| Night 3 | 3.8 at 05:30 | 8.3 | 4.5 h | 10.5 h |
| Day, 07:00 to 15:00 | 9.3 at 07:00 | 5.0 | 11.5 h | 1 h |
All three nights fall below 7, and each is lower than the one before because the model allows only 4.5 hours of day sleep between them. The day shift stays well above the line; its 11.5 hours of sleep are the last day sleep plus a 7-hour night.
How to read it, and its limits
Read the curve as a typical person on your schedule, not as you. The model was built from group averages and tested in laboratory and field studies, including a 2014 study of 136 airline crew members.
It does not know how you actually slept, and it leaves out naps, caffeine, light and workload. It also does not count time on task: a 12-hour night from 19:00 has the same predicted low as an 8-hour night from 23:00 if you got up at 07:00.
Use it to compare schedules and to see where the low hours fall. To record how shifts actually feel, rate them on the Samn-Perelli scale.
Common questions
- What does the KSS column mean?
- It estimates the Karolinska Sleepiness Scale rating at the lowest point: 10.6 minus 0.6 times predicted alertness, the conversion used in a 2014 validation study.
- Why does it assume 4.5 hours of day sleep?
- The model's authors found that sleep length falls to about 4.5 hours for bedtimes between morning and noon, because the body clock wakes you. Your own day sleep may be longer or shorter.
- Is my schedule sent anywhere?
- No. The calculator runs in your browser and sends nothing.
See your own shifts this way.
Bring in your schedule from any scheduling app with its calendar link. Capillary charts your hours, nights, quick returns and hardest shifts. Only you can see it.